Add check for input import before running FixInput

[Ionaru]

Check if from builtins import input is already present (result of raw_input fixer) and skip the input fixer if it does.

Fixes #427

[Ionaru]

The test TestFuturizeSimple::test_input_without_import fails on Py26, I don't know why.
@jmadler @anikamukherji any idea?

I wouldn't be against dropping Py26 support with #496 though 😛

[jmadler]

@Ionaru - The failure looks unrelated. I'd prefer we not drop Py26 support until absolutely necessary.

I'll merge it in and follow-up on the Py26 test failure.